Although research into the CO2 reforming of natural gas had been initiated in the 1920s, it gained renewed interest in the 1990s because of its potential applications in the greenhouse chemistry. It is a promising means of disposing and recycling two important greenhouse gasses, CH4 and CO2, and a route to producing valuable synthesis gas.
Compared with steam reforming or partial oxidation of methane, carbon dioxide reforming of methane provides synthesis gas with a relative low H2/CO ratio, which is more desirable for the direct use as feedstock for the synthesis of hydrocarbons, oxygenates, hydroformylation, oxo synthesis, and so on. Also, this reaction is usually considered as chemical energy transmission system (CETS) due to its strong endothermic characteristic, in which a power source generated from solar or nuclear energy drives this intensively endothermic reforming reaction and converts these inexpensive energies into valuable chemical energy.
However, the major drawback of this catalytic process up to now remains the rapid deactivation of the catalysts originating from the sintering of the metal active sites as well as the carbon deposition. Therefore, the recent research focus in this field has been mainly concentrated on developing catalysts with favourable capacity of anti-coke and anti-sintering.
Many kinds of catalysts using Ni or noble metals such as Ru, Rh, Pd, Ir and Pt have been reported to be active in this reaction. Noble metals have promising catalytic properties and low sensitivities to carbon deposits compared to Nickel however limited resources and high cost of noble metals limits their application in large-scale processes. Hence, Ni catalysts have been extensively investigated because of the metal availability and economic reasons.
Although Ni-based catalysts exhibit high activity and selectivity and are cheap, but the major drawback of this reaction, however, is the rapid deactivation of catalysts as a result of carbon deposition and sintering of Ni metal particles. Thus, in recent years, much effort has been devoted to developing Ni-based catalysts with improved performance, i.e. lower coke deposition and higher stability against metal sintering.
Former experimental and theoretical studies have confirmed that size of the Ni particle has a crucial role in suppressing coke. It has been reported that carbon deposition can occur only when the metal cluster is greater than a critical size. Therefore, to inhibit carbon deposition, it should be ensured that the size of the metal cluster is smaller than the critical size needed for coke formation.
Hence, many methods have been explored recently in order to obtain Mesoporous nanocrystalline powders with high surface areas for catalytic applications. They provide catalysts with more edges and corners, which can lead to higher performance.
Among the catalyst supports, magnesium aluminate spinel, MgAl2O4, has been widely used in industrial applications. This material has unique properties, such as high melting temperature (2135° C.), high mechanical strength at elevated temperature, high chemical inertness, good thermal shock resistance and catalytic properties Several synthesis methods for the preparation of MgAl2O4 spinel powders have been employed, such as: sol-gel, hydrothermal, combustion and co-precipitation. For many of its applications especially as catalyst support, a high surface area, small crystalline size, high porosity and more active sites, are more desired. Due to the low density and good thermal stability, it has long been used as catalyst support for catalytic reforming.
Also, the studies report that Ceria is an effective promoter to prevent the metallic sintering and to favour the activity as well as the resistance to coke formation. It is known for its high oxygen storage/transport capacity (OSC), i.e. its ability to use its lattice oxygen under oxygen poor environment and quickly reoxidize under oxygen rich environment. Thus, it increases nickel dispersion and enhances resistance towards sintering and coke formation.
Reference may be made to the article Applied Catalysis A, General 384 (2010) 1-9, by V. M. Gonzalez-Delacruz et al. where they got about 50%, 40%, 35% methane conversion at 750° C. temperature at the GHSV 3,00,000 L/kg h over Ni—CeO2 (26% Ni), Ni—CeO2 (13% Ni), Ni—CeO2 (7% Ni) catalyst. The main drawback of this process is the relative low conversion of methane (only 50%, 40%, and 35%) which decreases with time; furthermore, the GHSV of the process is too high.
Reference may be made to the article Applied Catalysis A: General 377 (2010) 16-26, by A. Kambolis et al. in which they got about 20-40% conversion of methane and 34-54% of carbon dioxide at 973° C. by using Ni/CeO2—ZrO2 catalyst. The main drawback of this process is the relative low conversion of both methane and carbon di oxide. Furthermore, 973° C. is high temperature.
Reference may be made to the article Catalysis Today 157 (2010) 436-439 by B. Koubaissy et al. in which they got 90% conversion of both the methane and carbon dioxide at 800° C. At a GHSV of 30 L h−1 g−1 by using Ce2Zr1.51Ni0.49Rh0.03 as a catalyst. The main drawback of this process is although Rh-based catalysts display unique efficiency and selectivity in catalysing dry reforming, but its high cost is a major problem for industrialisation.
Reference may be made to the article Catalysis Today 172 (2011) 226-231, by M. Ocsachoque et al in which they got 68%, 75%, 85% conversion respectively at 750° C. at a GHSV of 2,00,000 mL/hr./g by using the catalyst Ni/Al, Ni/Ce (3%) Al and Rh—Ni (3%)/Ce Al respectively. Although the conversion of both methane and carbon di oxide are quite appreciable however the GHSV of the reaction is too high, as well as in the third catalyst they used Rh, the high cost of Rh is the main problem for its industrialisation.
Reference may be made to the article Fuel Processing Technology 92 (2011) 1236-1243, by K. -M. Kang et al. in which they got 92% and 95% conversion of methane and carbon dioxide at 800° C. temperature by using Ni/Al2O3 as a catalyst and 92.5% and 91.8% conversion by using Ni/MgO—Al2O3 as catalyst.
Reference may be made to the article Fuel Processing Technology 92 (2011) 1868-1875, by T. Huang et al. in which they got 96% methane conversion at 4000 ml gcat−1h−1 GHSV, at 800° C. over (0.5%) Mo-(1%) Ni/SBA-15 catalyst. Although the reaction gives quite good methane conversion nevertheless, the authors did not provide any result about carbon dioxide.
Reference may be made to the article Journal of Natural Gas Chemistry Vol. 21 No. 2 2012, in which they got 70% methane and 74% carbon di oxide conversion at 700° C. at 18,000 mL/hr./g GHSV by using 7 wt. % Ni/MgAl2O4 catalysts. Although they got quite appreciable result however the GHSV of the reaction is too high.
Reference may be made to the article fuel processing technology vol. 119 (2014) 151-157, in which they got 74-81% methane conversion and 52-67% carbon dioxide conversion at 700° C. with the gas mixture CH4:H2O:CO2:N2=1:0.8:0.4:1 with a GHSV of 530,000 mL/h-gcat in a combined steam and carbon di oxide of methane by using Ni—CeMgAl2O4 catalyst with varying Ce:Ni ratio 0-1 and commercial MgAl2O4. Although they got quite good result but the surface area of the catalyst is too low as well as it doesn't gives good conversion for prolonged time as there is significant amount of coke deposition occurs during the reaction. By the use of different and improve process parameters to our catalyst (i.e., temperature, flow rates, feed compositions) provide more durable and better coke inhabitant catalyst system and the difference in the catalyst preparation method (i.e., mode of preparation of support, procedure to synthesis of Ni nanoparticles, size of the particles in the catalyst, the metal and support precursors used for synthesis etc.) which provide us a high surface area support and very small Ni nanoparticles. Combination of both the two parameters in the catalyst system enhances the coke resistivity and the stability of the catalyst system in aforementioned operation conditions.
The challenges of the process reported so far is that although they exhibit sufficiently high conversion of methane and high selectivity of syngas of unit H2/CO ratio but the rapid formation of coke causes deactivation of reforming catalyst. To overcome the deactivation of reforming catalyst many researchers used noble metals such as Pt, Ru, Rh etc. but the rising cost and relatively poor availability desiccates the use of those catalysts in industrial purpose. On this economic boundation, Ni based catalyst will be the holy grail for methane reforming in coming future. There is therefore, an evident necessity for further improvements in the Ni based catalyst and process for the dry reforming of methane with carbon di oxide.